1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device capable of improving image quality and aperture ratio, and a method for fabricating the same.
2. Discussion of the Related Art
Among the image display devices for displaying image data on their screens, a cathode ray tube (CRT) has been most widely used. However, the CRT is heavy and bulky relative to its display area. Thus, the use of the CRT is very inconvenient.
With the recent development of electronic industries, the display devices that have been restrictively used in TV screens are now used in personal computers, notebook computers, mobile terminals, dash boards, electronic display boards, and so on. In addition, with the development of information communication technologies, large capacity of image information can be transmitted. Therefore, next-generation flat panel display devices capable of processing and representing the large capacity of image information become increasingly important.
The next-generation flat panel display devices have to be lightweight and have a low cost slim profile, high brightness, large screen, and low power consumption. Among them, a liquid crystal display device (LCD) is in the spotlight.
The LCD has better resolution than other any flat panel display device. In displaying the moving pictures, the LCD has a rapid response time, which is comparable to that of the CRT.
As is well known, the LCD is driven using an optical anisotropy and polarization of liquid crystal.
Since the liquid crystal has a thin and long structure, it has directionality in a molecular arrangement. The direction of the molecular arrangement can be controlled by applying electric field to the liquid crystal.
Accordingly, if the direction of the molecular arrangement is arbitrarily controlled, the molecular arrangement changes and thus light polarized by the optical anisotropy is arbitrarily modulated. In this manner, the image information is displayed.
An active matrix LCD (AM LCD) has high resolution and good reproducibility of moving images. In the AM LCD, thin film transistors (TFTs) and pixel electrodes connected thereto are arranged in a matrix form.
A description of a process of forming an alignment layer that determines an initial arrangement direction of the liquid crystal molecule in the AM LCD follows.
First, a polymer thin film is coated and an alignment layer is arranged in a constant direction.
The alignment layer is generally formed of a polyimide-based organic material, and a rubbing method is widely used to arrange the alignment layer.
In the rubbing method, a polyimide-based organic material is coated on a substrate and is arranged after removing a solvent at 60-80° C. Then, the polyimide-based organic material is hardened at 80-200° C. to thereby form a polyimide alignment layer. The alignment layer is rubbed using a rubbing cloth around which velvet is wound. In this manner, various alignment directions are formed.
Since the rubbing method is convenient to the alignment process, it is suitable for mass production and can provide a stable alignment.
However, since the rubbing method is performed through the direct contact between the rubbing cloth and the alignment layer, cells may be contaminated by particles, and the TFTs on the substrate may be damaged. Also, after the rubbing, an additional cleaning process is required, and non-uniformity of the alignment may occur in the application to large-sized LCDs. Consequently, the yield of the LCDs is degraded.
To solve these problems of the rubbing method, non-rubbing techniques that do not use the mechanical rubbing method have been proposed.
Examples of the non-rubbing techniques include a method using Langmuir-Blodgett film (LB film), an optical alignment method using UV irradiation, a method using oblique deposition of SiO2, a method using micro-groove formed by photolithography, and a method using ion beam irradiation.
The method using the ion beam can solve the problems of the mechanical rubbing method and can use the existing alignment material as it is. Thus, this method can be suitably used in the large-sized LCD.
FIG. 1 is a schematic view of an ion beam irradiation apparatus used to form a related art alignment layer.
Referring to FIG. 1, the ion beam irradiation apparatus is divided into three regions, that is, a first region 103 in which injected gas is ionized into ions and electrons to thereby form plasma, a second region 106 through which the ions are accelerated in a beam, and a third region 111 in which the accelerated ion beam 110 is emitted and reaches a substrate 120.
The injected gas is ionized into ions in the region 103 in which the plasma is formed, and the ions are accelerated and irradiated onto the substrate 120.
That is, the ion beam irradiation apparatus includes an ion beam source 100, a vacuum vessel 140, and a holder 121. The ion beam source 100 has a cathode 101, an anode 102, and an ion beam emitting medium 104, and an ion beam accelerating medium 105. The vacuum vessel 140 allows the ion beam 110 generated from the ion beam source 100 to irradiate without significant deviation to the substrate 120. Also, the holder 121 fixes the substrate 120 at a constant angle within the vacuum vessel.
Although not shown, a shutter can be provided between the ion beam source 100 and the substrate 120 to control an irradiation time of the ion beam 110 with respect to the substrate 120.
The ion beam source 100 generates the ions and the ion beams. Due to the voltage difference between the cathode 101 and the anode 102, the injected gas is ionized to thereby generate the plasma containing the electrons and the ions. The ions contained in the plasma pass through the ion beam emitting medium 104 to thereby emit the ion beam 110.
The ion beam emitted from the discharged plasma is accelerated by the action of the electric field applied to the ion beam accelerating beam 105. Thus, the ion beam is irradiated onto the substrate 120 at a constant angle.
At this time, the substrate 120 is inclined at a predetermined angle with respect to the irradiated ion beam 110. Therefore, using the ion beam 110, a desired alignment direction can be formed on the alignment layer coated on the substrate 120, and a pretilt angle can be formed.
Like this, the ion beam 110 from the ion beam source 100 is emitted in a direction normal to the ion beam source 100, and the pretilt angle of the liquid crystal molecule is determined by an irradiation angle θ2 with respect to the alignment layer of the substrate 120 inclined at a predetermined angle θ1. Here, θ1=θ2.
The irradiation angle θ2 represents an angle between an irradiation direction of the ion beam 110 and the normal direction of the substrate 120. The relationship between the irradiation angle θ2 and the pretilt angle is shown in FIG. 2.
Referring to FIG. 2, the pretilt angle has different characteristic depending on the irradiation angle of the ion beam. The liquid crystal molecule has the maximum pretilt angle of 5° when the irradiation angle of the ion beam is in the range of 40-60°, and has the pretilt angle of below 5° when the irradiation angle is in the range out of 40-60°.
Therefore, in order to obtain the desired pretilt angle in the LCD, the ion beam with the appropriate irradiation angle must be irradiated with the same energy on the entire surface of the alignment layer of the substrate.
The pretilt angle, however, has the different characteristic depending on the irradiation angle of the ion beam, as shown in FIG. 2. Therefore, the uniform ion beam energy must be irradiated to obtain the desired pretilt angle.
To irradiate the uniform ion beam energy, the distance between the position from which the ion beam is emitted and the substrate must be sufficiently long. As the size of the substrate becomes larger, its length must be longer. Consequently, the size of the apparatus increases exponentially.
A twisted nematic (TN) LCD is widely used. In the TN LCD, electrodes are formed on two substrates, respectively. A liquid crystal director is arranged to be twisted at 90°. Then, the liquid crystal director is driven by applying a predetermined voltage electrodes.
However, the TN LCD has the greatest problem of a narrow viewing angle.
To solve the problem of the narrow viewing angle, new LCDs have been developed. Examples of the new LCDs are an in-plane switching (IPS) LCD and an optically compensated birefringence (OCB) LCDs.
In the IPS LCD, a common electrode and a pixel electrode are formed together in a pixel region of the same substrate to drive liquid crystal molecules in a state in which the liquid crystal molecules are maintained horizontally with respect to the substrate. By applying a predetermined voltage between the common electrode and the pixel electrode, an electric field is generated in a direction horizontal to the substrate. That is, a major axis of the liquid crystal molecule lies down instead of standing up.
For this reason, a variation in the birefingence of the liquid crystal with respect to the viewing direction is small, and thus the viewing angle of the IPS LCD is better than that of the TN LCD.
In the IPS LCD, however, a step is caused by the common and pixel electrodes and gate and data lines. Accordingly, when the ion beam is irradiated to arrange the alignment layer in a constant direction, the irradiated ion beam does not reach certain regions adjacent to the step, so that certain regions are not aligned.
The ion beam is inclined at a predetermined angle with respect to the substrate. Therefore, in the step region such as the common and pixel electrodes and the conductive lines, the ion beam is not irradiated to a side of the step and its adjacent region disposed opposite to the irradiation direction of the ion beam.
Meanwhile, the problem that the alignment characteristic is degraded in the step region also occurs in the TN LCD, the vertical alignment (VA) LCD, and the OCB LCD.
FIG. 3 is a partial sectional view illustrating a process of irradiating an ion beam onto an alignment layer formed on a common electrode and a pixel electrode in a related art IPS LCD.
Referring to FIG. 3, a common electrode 217 is formed on a substrate 200, and a gate insulating layer 251 and a passivation layer 253 are formed on the common electrode 217.
A data line 210 is formed between the gate insulating layer 251 and the passivation layer 253.
Although not shown in FIG. 3, a gate line is formed on the substrate 200. The gate line and the data line 210 cross each other to define a pixel region.
A pixel electrode 230 is formed on the passivation layer 253. The pixel electrode 230 is not overlapped with the common electrode 217. An alignment layer 255 is formed on the passivation layer 253 and the pixel electrode 230.
In the alignment process of the alignment layer 255 using the ion beam 245, a step difference is formed by the common electrode 217, the pixel electrode 230, the data line 210, and the gate line (not shown). Therefore, due to the taper angle of the step difference, the ion beam 245 irradiated at a specific angle does not reach regions A, B and C.
As illustrated in FIG. 3, when the ion beam 245 is irradiated at an angle θ with respect to the substrate 200, the ion beam does not reach the regions A, B and C due to the step difference between the electrode and the line. The alignment is not achieved in these regions A, B and C, and thus the liquid crystal is not arranged. Consequently, light leakage occurs in the pixel region, resulting in the degradation of the contrast ratio.
Also, light leakage occurs in the region in which the liquid crystal is not uniformly arranged because of the step difference of the conductive lines. A width of a black matrix increases to prevent the light leakage, and therefore an aperture ratio is degraded.